The invention disclosed and claimed herein pertains generally to an apparatus and method for optical heterodyning. More particularly, the invention pertains to an optical heterodyne system which provides an electrical representation of an image contained in the field or wavefront of a coherent light beam, such as the image of an object from which the beam has been reflected. Even more particularly, the invention pertains to an optical heterodyne system for visually observing an object by means of apparatus for, and the steps of receiving a first coherent light beam reflected from the object, heterodyning or mixing different discrete components of the first coherent light beam with a second coherent light beam of a different frequency, and receiving signals generated by the heterodyne process, each of the discrete components being mixed and each of the signals being received in the same preselected sequence.
In an optical heterodyne receiver system, a coherent light signal carrying information modulated thereupon is received by the system, and is mixed with a coherent local oscillator light signal of a different frequency to provide an optical heterodyne signal. The heterodyne signal, having a frequency equal to the difference between the frequencies of the received and local oscillator signals, is transduced by a photo-detector to an electrical heterodyne signal. The carried information is then extracted from the electrical signal by means of well known electrical communications apparatus.
In the construction of an optical heterodyne receiver of the above type, it is very important to consider the angular alignment between the wavefronts of the received and local oscillator signals at the photodetector. If the alignment is not within a critical limit, mixing efficiency will be so degraded that information carried by the received signal will be undetectable. In the past, this problem has been overcome by focusing the field, or wavefront, of the received information carrying signal within a diffraction-limited spot, or Airy disc, and illuminating the disc with the local oscillator signal. Mixing occurs within the disc, and the light within the disc, comprising the aforementioned optical heterodyne signal, is sensed by a photodetector to provide an electrical signal representing the optical heterodyne signal. The dimensions of the Airy disc are selected in respect to the wavelength of the received information carrying signal so that most of the received signal energy is concentrated therewithin. Consequently, even though the received and local oscillator signals are misaligned, mixing efficiency within the disc is sufficiently high to provide a detectable heterodyne signal.
The above technique is exemplified in U.S. Pat. No. 3,426,207, 1969, to D. F. Fried et al, which discloses the use of the photocathode of an image dissector tube as the required photodetector. The detector plane of the photocathode is illuminated with light of the local oscillator signal, and the information carrying signal is focused in an Airy disc upon a small area of the detector plane. A stream of electrons, representing the light impinging upon a specified section of the detector plane, is directed from the photocathode to an electron detector grid. In order to maximize signal-to-noise ratio, it is necessary to ensure that the Airy disc continuously falls within such section. However, due to atmospheric or other factors affecting the transmission of the information carrying signal, the Airy disc tends to "wander" or randomly move on the detector plane of the photocathode. Consequently, some kind of tracking means must be provided to continuously monitor the position of the disc on the detector plane so that the planar section from which electrons are received by the detector grid is always the section upon which the Airy disc impinges. Such tracking means has generally included fairly complex electronic circuitry.
In addition to introducing the problem of wander, the above technique would have little or no applicability in an optical heterodyne imaging system, which is intended to provide the image of an object by receiving a coherent light beam reflected from the object. For example, the image of an object from which a coherent light signal is reflected is contained within, or carried upon, the field or wavefront of the signal, and may be thought of as comprising an array of discrete picture elements, each picture element having a discrete light intensity and position in the image. In order to view the image, it would be necessary to detect the magnitude and position of each such picture element. However, the above technique makes no provision for such detection. In addition, if the above technique was used in an imaging application, the gain in mixing efficiency provided by focusing the received wavefront would be lost.